The invention relates to a process for preparation of treatment gas used in heat treatments, whereby the treatment gas is produced in a furnace disposed catalyst retort at a temperature of that of the furnace. The invention also relates to a corresponding device for carrying out the inventive process. The phrase "catalyst retort at a temperature of that of the furnace" means that the retort temperature is at or near the temperature level of the inside of the furnace.
There are known many heat treatment processes, especially processes for heat treatment of metal workpieces, which involve the use of a catalyst retort placed in the hot areas of a heat treatment furnace for the production of a treatment gas (a protective or reaction gas). See, e.g., DE-OS 36 30 833. In operation, a feed gas is fed to these retorts wherein, due to the presence of the catalyst and the existing temperature level imposed by the furnace environment, the feed gas is reacted to the desired treatment gas. The resultant treatment gas then flows directly from the catalyst retort into the furnace interior.
However, in heat treatments wherein the ambient temperature for the catalyst retort is below 800.degree. C., problems can occur with respect to complete reaction of the feed gas due to the relatively low temperature level. These problems can be eliminated, within certain bounds, in self-heated catalyst retorts by raising the heat output of a burner positioned inside the catalyst retort and adjacent to the catalyst material. But, at a certain lower temperature limit, about 750.degree. C., thorough warming of the catalyst mass becomes insufficient because of excessive heat outflow from the retort to the "too cold" furnace space. This in turn causes the reaction of the feed gas into the treatment gas to be insufficient.
On the other hand, if heating output is further increased, there is a danger that a burning through of the catalyst retort will occur. The catalyst retort, in general, is designed as a pipe with catalyst material placed on the outside thereof and a heating burner positioned on the inside thereof. Such tubular catalyst retorts equipped with their own heating means are known from, e.g., DE-OS 27 58 024 or DE-OS 36 32 577.
Moreover, with such catalyst retorts there is the problem that in situations where replacement of the retort becomes necessary a temporary shutdown of the furnace has to take place.